A 35S subunit has been identified in mouse cells with properties of a replication-defective endogenous type-C virus. This represents the second species in which this class of virus has been recognized. A polypeptide, 50,000 mw has been identified as the 5' product of this RNA. This protein is the first marker for this class or retravirus. It may provide a marker for naturally occurring tumors not previously available. This type of defective virus is a unique class of endogenous type-C virus and may be the progenitor for transforming genes in naturally occurring cancer.